1. Field of the Invention
The present invention relates to the design of semiconductor light-emitting devices. More specifically, the present invention relates to a semiconductor light-emitting device with an electrode that can form a low-resistance conductive path to an N-polar surface of an InGaAlN layer.
2. Related Art
Solid-state lighting is expected to be the illumination wave of the future. High-brightness light-emitting diodes (HB-LEDs) are beginning to penetrate an increasing number of applications, especially as light source for display devices and as light-bulb replacement for conventional lighting. Operation voltage, along with brightness and efficiency, are essential performance metrics for LEDs.
An LED typically produces light from an active region, which is situated between a positively-doped layer (p-type doped layer) and negatively-doped layer (n-type doped layer). When the LED is forward-biased, the carriers, which include holes from the p-type cladding layer and electrons from the n-type cladding layer, recombine in the active region. For direct band-gap materials, this recombination process releases energy in the form of photons, or light, the wavelength of which corresponds to the band-gap energy of the material in the active region.
The recently developed group III nitride (InxGayAl1-x-yN, 0<=x<=1, 0<=y<=1)-based LEDs not only extend the LED emission spectrum to the green, blue, and ultraviolet region, but also can achieve high light emission efficiency. InGaAlN-based LEDs thus have significantly expanded the application field of LEDs. In the following description, “InGaAlN” or “GaN” material generally refers to the Wurtzite InxGayAl1-x-yN (0<=x<=1, 0<=y<=1) materials, which can be a binary, ternary, or quaternary compound, such as GaN, GaAlN, InGaN, and InGaAlN.
A key property of the Wurtzite group III nitrides is their large spontaneous and piezoelectric polarization, which can significantly influence a device's electrical characteristics. The two polarities exhibited by an InGaAlN layer are the Ga-face polarity and N-face polarity, which are also referred to as the (0001) surface and (0001) surface, respectively. Generally, the InGaAlN epitaxial layers are formed using a chemical vapor deposition (CVD) process. During the CVD process, the growth surface facing away from the substrate typically exhibits a Ga-face polarity, regardless of the substrate material. Hence, the Ohmic-contact electrodes are mostly based on materials which can form a low-resistance conductive path to the Ga-polar surface of an InGaAlN layer.
In recent years, researchers have been experimenting with wafer-bonding techniques to construct vertical-electrode LEDs. During wafer bonding, a second support wafer is bonded to the top of the LED multilayer structure, and the initial growth substrate on which the device is epitaxially formed is removed. The entire device is then “flipped” upside-down. As a result, the GaN layer which is previously at the “bottom” of the device is now at the “top.” An Ohmic-contact electrode is then deposited on the GaN layer on the top. However, the surface of the flipped GaN layer which is exposed after the initial growth layer is removed typically exhibits an N-face polarity and has drastically different Ohmic-contact characteristics than a Ga-polar surface. For example, Al, a common electrode material used for a Ga-polar surface, can actually form an AlN layer and consequently a depletion region when in contact with an N-polar surface. This depletion layer can increase the potential barrier at the metal-semiconductor interface and raise the turn-on voltage threshold of the LED.
Hence, what is needed is an electrode capable of forming a low-resistance conductive path to the N-polar surface of an InGaAlN layer to produce a desirable Ohmic-contact which does not significantly increase the LED's turn-on voltage.